Structure of magnetostrictive shaft applicable to magnetostriction-type torque sensor for detecting torque applied to rotatable shaft and method for manufacturing the same

ABSTRACT

A structure of a magnetostrictive shaft applicable to a magnetostriction-type torque sensor and a method for manufacturing the same can achieve a high torque sensitivity. A shaft parent material is made of a non-magnetic austenitic series metallic material (YHD50) and a magnetic thin film is made of a magnetostriction material such as Iron-Aluminum series alloy plasma spray coated on the whole outer peripheral surface of the shaft parent material. In a second embodiment, a mechanical working is carried out for the shaft material on the surface of which the magnetostriction material thin film is coated, the worked shaft material is heated under an inert gas atmosphere, and the heated shaft material is immersed into oil under the inert gas atmosphere to perform oil quenching.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a structure of a magnetostrictive shaftapplicable to a magnetostriction type torque sensor which is soconstructed as to detect a torque applied to a rotatable shaft andoutput a signal according to a magnitude and direction of the appliedtorque and a method for manufacturing the same.

2. Description of the Background Art

U.S. patent applications Ser. No. 07/969,056 filed on Oct. 30, 1992,Ser. No. 08/068,668 filed on May 28, 1993, and Ser. No. 08/222,809 filedon Apr. 5, 1994 exemplify a previously proposed structure of amagnetostriction type torque sensor, the sensor inserted in an electricbridge circuit to output a signal according to a magnitude and directionof the torque applied to a magnetostrictive shaft interposed betweenintermediate ends of a torque applied rotatable shaft.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a structure of amagnetostrictive shaft applicable to a magnetostriction-type torquesensor and a method for manufacturing the same which can achieve a hightorque detection sensitivity.

The above-described object can be achieved by providing a structure of amagnetostriction-type torque sensor, comprising: a) a cylindrical outercasing; b) a magnetostrictive shaft disposed within said outer casing soas to enable a rotation thereof according to a torque applied thereto;and c) a pair of self-inductance determining coils, each coil beingconnected to a torque magnitude and direction detection circuitry andbeing wound around a radial part of an outer peripheral surface of themagnetostrictive shaft with a gap provided against the outer peripheralsurface, wherein said magnetostrictive shaft comprises a shaft parentmaterial made of a non-magnetic austenitic series metallic material anda magnetic thin film formed on the whole outer peripheral surface ofsaid magnetostrictive shaft parent material, said magnetic thin filmbeing made of a magnetostriction material.

The above-described object can also be achieved by providing a methodfor manufacturing a magnetostrictive shaft of a magnetostriction-typetorque sensor, said magnetostrictive shaft being interposed betweenintermediate ends of a rotatable shaft and having an outer peripheralsurface being made of a magnetostriction material, comprising the stepsof: a) mechanical working a shaft material of the magnetostrictiveshaft; b) heating said mechanically worked shaft material under an inertgas atmosphere and holding said mechanically worked shaft material at aheated temperature; and c) immersing the heated shaft material into anoil filled bath tinder the inert gas atmosphere so as to increase acooling rate of the heated shaft material.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partially cross sectioned view of a magnetostriction-typetorque sensor to which a first embodiment of a magnetostrictive shaftaccording to the present invention is applicable.

FIG. 2 is a partially enlarged and cross sectioned view of themagnetostrictive shaft of a previously proposed torque sensor forexplaining directions of magnetic fluxes in the case of a previouslyproposed magnetostrictive shaft shown in FIG. 1.

FIG. 3 is a simplified series of processes of manufacturing thepreviously proposed magnetostrictive shaft.

FIG. 4 is an example of an electric detection circuitry for detecting atorque applied to a rotatable shaft connected to the magnetostrictiveshaft and outputting a voltage signal E according to the torque appliedthereto.

FIG. 5 are characteristic graphs of hysterisis loops of the previouslyproposed magnetostrictive shaft and of a magnetostrictive shaft in thefirst embodiment according to the present invention.

FIG. 6 is a partially enlarged and cross sectioned view of themagnetostrictive shaft in the case of the first embodiment.

FIG. 7 is a simplified series of processes to manufacture and achievethe magnetostrictive shaft in the first embodiment.

FIG. 8 is a simplified series of processes to manufacture and achievethe magnetostrictive shaft in a previously proposed method formanufacturing the same.

FIG. 9 is a process of immersing a parent material of themagnetostrictive shaft in an oil-filled bath in the case of thepreviously proposed manufacturing method explained with reference toFIG. 8.

FIG. 10 is a characteristic graph of a torque-versus-output voltage inthe case of the previously proposed manufacturing method of themagnetostrictive shaft shown in FIG. 9.

FIG. 11 is a simplified series of processes of a method formanufacturing the magnetostrictive shaft in a case of a secondembodiment according to the present invention.

FIG. 12 is a schematic cross sectioned view of an oil bath under theinert gas (Argon) atmosphere for explaining an immersing process 500 inthe case of the second embodiment shown in FIG. 11.

BEST MODE FOR CARRYING OUT THE INVENTION

Reference will hereinafter be made to the drawings in order tofacilitate a better understanding of the present invention.

(First Embodiment)

Before explaining a first preferred embodiment of a structure of amagnetostrictive shaft applicable to a magnetostriction type torquesensor according to the present invention, the structure of a previouslyproposed torque sensor will be described below to which the presentinvention is applicable.

FIG. 1 shows an example of the torque sensor to which the firstembodiment of the magnetostrictive shaft is applicable.

It is noted that the structure of the torque sensor is exemplified by aU.S. patent application Ser. No. 07/969,056 filed on Oct. 30, 1992, thedisclosure of which is herein incorporated by reference.

In FIG. 1, a cylindrical outer casing 1 is formed around amagnetostrictive shaft 2 and is made of a non-magnetic material. Thecylindrical outer casing 1 is fixed on a casing of an automatic powertransmission unit of an automotive vehicle (not shown). Themagnetostrictive shaft 2 is rotatably disposed within the outer casing 1via ball bearings 3 and 3.

The magnetostrictive shaft 2 generally includes: a shaft parent (alsocalled mother) material 2A formed in a bar shape of a structural steelsuch as Carbon Steel (SC), Nickel-Chromium Steel (SNC),Nickel-Chromium-Molybdenum Steel (SNCM), Chromium Steel (SCr),Chromium-Molybdenum Steel (SCM), Manganese Steel (SMn), orManganese-Chromium Steel (SMn C); and a magnetic thin film 2B formedover a whole outer peripheral surface of the magnetostrictive parentmaterial 2A of a magnetostrictive material such as an Iron-Aluminiumseries alloy. Both ends of the magnetostrictive shaft 2 are projectedtoward the outside of the outer casing 1 so as to form an output shaft.A slit forming portion 2C is located on an axial intermediate portion ofthe magnetostrictive shaft 2 and formed on the magnetic thin film 2B.One group 4 of slit grooves are inscribed on a part of the outerperipheral surface of the slit forming portion 2C at each groove tiltangle of 45 degrees downward to the center peripheral surface. The othergroup 5 of slit grooves are inscribed on a part of the outer peripheralsurface of the slit forming portion 2C at each groove tilt angle of 45degrees upward from the center peripheral surface, as shown in FIG. 1.

Each slit groove 4 and 5 is spaced apart from each other with apredetermined interval of distance and is formed individually on thewhole radial outer peripheral surface with an equal interval of distancewith respect to the adjacent grooves. Each slit groove of the one group4 is formed with a first magnetic anisotropy portion 6 and each slitgroove of the other group 5 is formed with a second magnetic anisotropyportion 7. Each magnetic anisotropy portion 6 and 7 forms a magneticpath as denoted by broken lines of FIG. 2 due to a surface magneticfield.

Referring back to FIG. 1, a core member 8 encloses the slit formingportion 2C from a radial outside of the magnetostrictive shaft 2 and isformed in a stepped cylindrical shape of a magnetic material such as aniron. Detection/excitation coils 9 and 10 are disposed in an innerperipheral side of the core member 8.

The detection/excitation coils 9 and 10 are faced radially against themagnetic anisotropy portions 6 and 7. The detection/excitation coils 9and 10 are disposed in the inner side of the core member 8 viacorresponding coil bobbins (not shown). The coils 9 and 10 serve toenergize the coils and detect the torque applied to the shaft 2 when analternating voltage V is applied thereacross from an oscillator 13 aswill be described later.

As shown in FIG. 4, the detection/excitation coils 9 and 10 have theirself-inductances L1 and L2 and their iron losses and direct currentresistances are denoted by r1 and r2, respectively.

FIG. 4 shows a previously proposed detection circuitry applicable to thetorque sensor in the first preferred embodiment.

In addition, the torque detection circuitry to which the coils 9 and 10are connected is exemplified by a U.S. patent application Ser. No.08/068,668 filed on May 28, 1993, the disclosure of which is hereinincorporated by reference.

As shown in FIG. 4, the detection circuitry 11 includes: a bridgecircuit 12; the oscillator 13; a differential amplifier 14; and asynchronization/waveform detection processing circuit 15. The bridgecircuit 12 shown in FIG. 4 has four arms, the first arm having onedetection/excitation coil constituted by the self-inductance L1 anddirect current resistance r1 (iron loss inclusive), the second armhaving the other detection/excitation coil constituted by theself-inductance L2 and direct current resistance r2 (iron lossinclusive), the third arm having one variable resistor R, and the fourtharm having the other variable resistor R. A junction denoted by a ofFIG. 4 is connected to the oscillator 13 and a junction denoted by b isgrounded. The oscillator 13 outputs the alternating voltage V having afrequency of f. A junction c and a junction d are connected to a plusinput end of the differential amplifier 14 and to a minus input endthereof, respectively. It is noted that an output end of the oscillator13 is connected to the synchronization/waveform detection processingcircuit 15 and an output end 14A of the differential amplifier 14 isconnected to the above-described synchronization/waveform detectionprocessing circuit 15. The synchronization/waveform detection processingcircuit 15 synchronizes the output voltage E0 from the differentialamplifier 14 with the alternating voltage V and rectifies the outputvoltage E0 to output a direct-current output voltage E.

FIG. 3 shows a simplified flowchart of a previously proposedmanufacturing method of the magnetostrictive shaft 2 shown in FIG. 1.

At the process I of FIG. 3, the shaft parent material 2A constituted bythe structural steel is heated up to a temperature, for example, rangingfrom 750° C. to 900° C. to undergo a hardening so as to provide theshaft parent material 2A for a desired hardness.

At the next process II of FIG. 3, a spray coating of themagnetostriction material such as the iron-aluminium series alloy istreated on the whole peripheral surface of the shaft parent material 2Aso as to form integrally the magnetic thin film 2B with the shaft parentmaterial 2A.

At the next process III of FIG. 3, a surface treatment is carried out onthe magnetic thin film 2B over the whole outer peripheral surface of theshaft parent material 2A and a mechanical working of each slit groove 4and 5 is carried out to form the magnetic anisotropy portions 6 and 7 onthe surface of the magnetic thin film 2B.

At the next process IV of FIG. 3, a magnetic annealing is carried out byheating the whole magnetostrictive shaft 2 formed with the slit grooves4 and 5 up to about 850° C. so that a working distortion (strain) iseliminated from the magnetostrictive shaft 2, the degree of hysterisisis reduced, and output sensitivities and hysterisis deviations of theindividual magnetostrictive shafts 2 are reduced.

Next, the operation of the torque sensor shown in FIG. 1 and detectioncircuit shown in FIG. 4 will be described.

When the alternating voltage V from the oscillator 13 is applied to theexcitation/detection coils 9 and 10, the magnetic paths due to thesurface magnetic fields are formed along the magnetic anisotropyportions 6 and 7 between the respective slit grooves 4 and 5 of the slitforming portion 2C. In this case, the variable resistors R are adjustedto provide zero output voltage E0 of the differential amplifier 14 withthe torque applied to the magnetostrictive shaft 2 given as zero.

When, in this state, a torque T denoted by an arrow marked direction ofFIG. 1 is acted upon the shaft 2, a tensile stress+σ is acted along theone magnetic anisotropy portion 6 between each slit groove 4 and acompressive stress-σ is acted along the other magnetic anisotropyportion 7 between each slit groove 5. If a positive magnetostrictionmaterial is used for the magnetostrictive shaft 2, the magneticanisotropy portion 6 increases its permeability μ due to the tensilestress+σ and, on the contrary, the magnetic anisotropy portion 7decreases its permeability μ due to the compressive stress-σ.

Here, since the detection/excitation coil 9 disposed so as to faceagainst the magnetic anisotropy portion 6 of the shaft 2 has theself-inductance L1 which is increased due to the increase in thepermeability μ, the current flowing through the coil 9 is decreased. Onthe other hand, since the self-inductance L2 of the coil 10 is decreaseddue to the decrease in the permeability μ, the current flowing throughthe coil 10 is increased. Consequently, the detected voltage V1 from thecoil 9 is decreased and the detected voltage V2 from the coil 10 is, inturn, increased, the differential amplifier 14 carries out theamplification of the difference between V1 and V2 as follows:

E0=A×(V1-V2)--(1), wherein A denotes an amplification factor.

The alternating output voltage E0 from the output end of thedifferential amplifier 14A is transmitted to thesynchronization/waveform detection processing circuit 15. Thesynchronization/waveform detection processing circuit rectifies andprocesses the output voltage E0 so that the direct-current outputvoltage E, as shown in FIG. 5, is output as the detection signalcorresponding to the magnitude and direction of the torque applied tothe shaft 2.

A relationship between the torque acted upon the shaft 2 and outputvoltage E is represented by a characteristic curve 16 of FIG. 5. Thetorque sensitivity is represented by a gradient θ and the hysterisis ofthe shaft when the torque is varied is represented as a hysterisis widthh of the curve 16 in FIG. 5.

In the previously proposed magnetostrictive shaft 2, the magnetic pathsfrom the coils 9 and 10 are invaded into the internal of the shaftparent material 2A, as denoted by the broken lines of FIG. 2, so that aninternal magnetic flux of the magnetic thin film becomes low due to theuse of the structural steel of the parent material of the shaft 2A.

However, in the first embodiment, the magnetic paths are concentratedinto the internal of the magnetic thin film 2B of the magnetostrictiveshaft 2 so that the magnetic flux can be high and the torque detectionsensitivity can be improved.

In the first embodiment, the shaft parent material of themagnetostrictive shaft is formed of a non-magnetic austenitic seriesmetallic material so that the magnetic fluxes from thedetection/excitation coils are not invaded into the shaft parentmaterial and the magnetic flux density of the magnetic thin film can bepositively higher.

FIG. 6 and FIG. 7 show the first preferred embodiment of the structureof the magnetostrictive shaft 21 according to the present invention.

In FIG. 6, the magnetostrictive shaft 21 used in the first embodimentincludes the shaft parent material 21A and magnetic thin film 21B in thesame way as described above.

However, in the first embodiment, the parent material of the shaft 21Ais formed of a non-magnetic austenitic series metallic material such asa hot working die steel (YHD50).

Here, the manufacturing method of the magnetostrictive shaft 21 will beexplained with reference to FIG. 7.

At a process of (1) in FIG. 7, the shaft parent material 21A made ofYHD50 is previously prepared.

Then, a plasma spray coating with a powder of a magnetostrictionmaterial such as an iron-aluminium series alloy is carried out for thewhole peripheral surface of the shaft parent material 21A so as to formthe thin magnetic film 21B over the whole periphery of the parent shaftmaterial 21A. In this case, since the shaft parent material 21Aconstituted by YHD50 is substantially hardened under a heating processof (3) as will be described below, the hardening process of process Ishown in FIG. 3 can be omitted.

A thickness of the magnetic thin film 21B approximately corresponds to askin depth S expressed as follows:

S=√(2ρ/Wμ), wherein ρ denotes an electrical resistance of the magneticthin film, W denotes an external magnetic field angular velocity, and μdenotes its permeability. For example, the thickness t is set to a rangefrom 0.1 mm to 0.3 mm.

Next, at a process (2) of FIG. 7, a surface working on the magnetic thinfilm 21B is carried out and a mechanical working to form the slitgrooves (denoted by 4 and 5 in FIG. 1) of the respective magneticanisotropy portions is carried out on the surface of the magnetic thinfilm 21B.

Next, at a process (3) of FIG. 7, the magnetostrictive shaft 21 isheated up to a temperature range from 700° C. to 1100° C. and its heatedstate is held for a period of time equal to or more than one hour.Thereafter, a magnetic annealing is carried out by quenching (cooling)the heated parent material 21A into an oil (oil-filled bath) so that aworking strain (worked distortion) is eliminated, the hysterisis widthis reduced, and deviations in the hysterisis loops and outputsensitivities for the individual magnetostrictive shaft products arereduced. At this time, the shaft parent material 21A is substantiallyhardened to increase the hardness thereof.

The magnetostrictive shaft 21 is made as described above, i.e., theshaft parent material 21A being formed of the non-magnetic austeniticseries metallic material such as YHD50 and the plasma spray coating withthe magnetostriction material such as the iron-aluminium series alloy iscarried out to form the magnetic thin film 21B.

Therefore, as shown in FIG. 6, the magnetic fluxes from the coils arenot invaded into the inside of the parent material 21A of the shaft 21so that the magnetic fluxes are concentrated into the magnetic thin film21B of the shaft 21 and the magnetic flux density can be high.Consequently, the torque detection sensitivity of the magnetostrictiontype torque sensor can be improved. Thus, the gradient θ₁ of thecharacteristic curve 22 denoted by a broken line of FIG. 5 can beenlarged.

In addition, since the shaft parent material 21A is formed of YHD50, themagnetic annealing for the magnetic thin film 21B and hardening for theshaft parent material 21A are simultaneously carried out at the process(3) of FIG. 7 under the same heating process. The manufacturing methodof the magnetostrictive shaft 21 can be simplified and the manufacturingcost can be reduced.

The magnetostriction material to be used during plasma spray coating forthe shaft parent material 21A is made of an alloy such as a permalloy.

(Second Embodiment)

FIG. 8 shows a previously proposed manufacturing method of themagnetostrictive shaft 2.

At a process of (I) of FIG. 8, the shaft parent material 2A constitutedby the structural steel is hardened to heat the parent material of theshaft 2 up to a temperature ranging from, for example, 750° C. to 900°C. to provide the shaft parent material 2A with the desired hardness.

At the next process of (II), the magnetostriction material such as theiron-aluminium series alloy is spray coated on the whole peripherysurface of the shaft parent material 2A so that the magnetic thin film2B is integrally formed on the parent material 2A of the shaft 2 to formintegrally the shaft material 16.

At the next process of (III), the surface working is carried out for themagnetic thin film 2B enclosing the whole surface of the outerperipheral surface of the shaft parent material 2A and the mechanicalworking is carried out to form the slit grooves 4 and 5 on the surfaceof the magnetic thin film 2B so as to form the pair of magneticanisotropy portions 6 and 7.

At the next process of (IV), the whole shaft 16 formed with the slitgrooves 4 and 5 are heated up to, for example, 800° to 900° C.,preferably, at about 850° C. under the air or nitrogen gas atmosphereand held at the heated temperature for about one hour.

Next, at the process of (V), the whole shaft material 16 heated underthe heating process (IV) is oil quenched by immersing the whole shaftmaterial 16 into the oil bath 17 as shown in FIG. 9 under the air(atmospheric pressure) or nitrogen gas.

Thus, at both processes of (IV) and (V), the magnetic annealing iscarried out for the shaft material 16 to produce the shaft material 16.

FIGS. 11 and 12 show a second embodiment of the method for manufacturingthe magnetostrictive shaft 2.

First, at processes 100 through 300, the working process is shown. Thisworking process is the same as that in the processes of (I) through(III) shown in FIG. 8. At the process of 100, the shaft parent material2A constituted by the structural steel is heated up to, for example,750° C. to 900° C. to perform the hardening.

At the process of 200, the magnetostriction material such as theiron-aluminium series alloy is spray coated over the whole periphery ofthe shaft parent material 2A to form integrally the magnetic thin film2B with the shaft parent material 2A, thus forming the shaft material21.

At the process of 300, the surface working is carried out for themagnetic thin film 2B enclosing the whole surface of the parent shaftmaterial 2A and the mechanical working is carried out on the magneticthin film 2B to form the slit grooves 4 and 5.

Next, at a process of 400 of FIG. 11, with an inert gas having 0.067 to0.080 MPa (,i.e., 500 through 600 Torr) and constituted by an Argon (Ar)gas (or alternatively, Helium gas or Neon gas) filled in an air-sealedchamber 22 shown in FIG. 12, the shaft material 21 is heated by means ofan electrical furnace (not shown) up to a temperature, for exampleranging from 800° C. to 900° C., preferably, about 850° C. and held atthe heated temperature for about one hour.

Next, at a process of 500, the whole heated shaft material 21 at theprocess of 400 is immersed into the oil 17 in the air-sealed chamber 22with the Argon gas filled as shown in FIG. 12 so that the oil quenchingis carried out for the shaft material 21 to achieve the magneticannealing.

However, in the second embodiment, the heating of the shaft material 21is performed for about one hour at about 850° C. with the Argon gasfilled under the atmospheric pressure in the air-sealed chamber 22. Inthe oil quenching process, the whole shaft material 21 heated at theheating process is oil quenched into the oil 15 of the air-sealedchamber 22 filled with the Argon gas to perform the magnetic annealing.

Since the Argon gas is filled in the air-sealed chamber 22 with theshaft material 21 heated at the hearing process, the surface of themagnetic thin film 2B cannot be oxidized and nitrided when the shaftmaterial 21 is held in the heated state. Hence, the hysterisis of themagnetostrictive shaft cannot be increased.

In addition, since the shaft material 21 is immersed into the oil 17under the Argon gas atmosphere, the cooling velocity of the shaftmaterial 21 can be fastened. Consequently, a large detection sensitivityto the magnetostrictive shaft 2 can be obtained.

Weight contents of Aluminium in the Iron-Aluminium series alloyconstituting the magnetic thin film 2B are varied as 10 wt %, 13 wt %,and 15 wt %. At this time, the sensitivities and numerical values of thehysterisis were indicated as below.

It is noted that as reference values numerical values when the heatingprocess (400) and (500) were carried out were indicated as well.

                  TABLE 1                                                         ______________________________________                                        Fe--Al alloy    Fe--Al alloy Fe--Al alloy                                     (Al 10 wt %)    (Al 13 wt %) (Al 15 wt %)                                     Sens.       Hys     Sens.    Hys   Sens. Hys                                  (V)         (%)     (V)      (%)   (V)   (%)                                  ______________________________________                                        Ar      1.963   0.80    1.573  0.16  1.340 0.53                               Vacuum  1.157   0.87    0.830  0.36  0.891 0.66                               Air     --      --      0.232  3.2   --    --                                 ______________________________________                                    

Consequently, as shown in the above-listed Table 1, the hysterisis ofthe manufactured magnetostrictive shaft can be reduced and the torquedetection sensitivity of the sensor manufactured in the series ofprocesses of the second embodiment can be higher.

While the present invention has been disclosed in terms of the preferredembodiment in order to facilitate better understanding thereof, itshould be appreciated that the invention can be embodied in various wayswithout departing from the principle of the invention. Therefore, theinvention should be understood to include all possible embodiments andmodifications to the shown embodiments which can be embodied withoutdeparting from the principle of the invention as set forth in theappended claims.

What is claimed is:
 1. A structure of a magnetostriction-type torquesensor, comprising:a) a cylindrical outer casing; b) a magnetostrictiveshaft disposed within said outer casing so as to enable a rotationthereof according to a torque applied thereto; and c) a pair ofself-inductance determining coils, each coil being connected to a torquemagnitude and direction detection circuitry and being wound around aradial part of an outer peripheral surface of the magnetostrictive shaftwith a gap provided against the outer peripheral surface, wherein saidmagnetostrictive shaft comprises a shaft parent material made of anon-magnetic austenitic series metallic material and a magnetic thinfilm formed on the whole outer peripheral surface of said shaft parentmaterial, said magnetic thin film being made of a magnetostrictionmaterial.
 2. A structure of a magnetostriction-type torque sensor asclaimed in claim 1, wherein said magnetic thin film is made of anIron-Aluminium series alloy.
 3. A structure of a magnetostriction-typetorque sensor as claimed in claim 2, wherein said magnetic thin film ismade by plasma spray coating the shaft parent material.
 4. A structureof a magnetostriction-type torque sensor as claimed in claim 3, whereina depth t of the magnetic thin film corresponds to a skin depth Sexpressed as S=√(2ρ/Wμ), wherein ρ denotes an electrical resistance, Wdenotes an external magnetic field angular velocity, and μ denotes apermeability of the magnetic thin film.
 5. A structure of amagnetostriction-type torque sensor as claimed in claim 4, wherein saidskin depth is set in a range from 0.1 mm to 0.3 mm.
 6. A structure of amagnetostriction-type torque sensor as claimed in claim 3, wherein saidmagnetic thin film is provided with a pair of magnetic anisotropyportions by mechanically working the magnetic thin film and which arelocated on the radial part of the outer peripheral surface facingagainst the pair of self-inductance determining coils with the gap, eachmagnetic anisotropy portion having its permeability varied according toa tensile stress applied to the magnetostrictive shaft.